1. Field of the Invention
The present invention relates to a rotor of an electric motor. The present invention also relates to a method of manufacturing a rotor of an electric motor.
2. Description of the Related Art
In a rotor of an electric motor having a permanent-magnet field system (e.g., a synchronous motor), it is known that a plurality of permanent magnets are mounted in an embedded manner at predetermined positions in a cylindrical rotor core formed by stacking or laminating magnetic sheets such as silicon steel plates. The rotor of this type is typically manufactured by preliminarily forming a plurality of magnet-retaining apertures (typically, axially-extending through holes) in a predetermined circumferentially-distributed arrangement about a rotor shaft, each aperture having a profile substantially corresponding to a profile of each permanent magnet; inserting respectively the plurality of permanent magnets into the magnet-retaining apertures; and fixing each permanent magnet by an adhesive or an impregnant.
In the above-described conventional method for manufacturing a rotor, in the case where the adhesive is used for fixing the magnets, relatively complicated manual operations, such as the application of the adhesive to the permanent magnets, the removal of the surplus adhesive leaking from the magnet-retaining apertures, and the like, may be required. Also, in the case where an impregnant is used for fixing the magnets, relatively large-scale equipment tends to be required for an immersion process for immersing the rotor core in the impregnant, a heating process for solidifying the impregnant, etc., and thus the number of manufacturing steps tends to increase. On the other hand, it is also known that, as a rotor manufacturing method capable of resolving the above inconveniences, the rotor core is previously fabricated so that predetermined gaps are formed between respective magnet-retaining apertures and respective permanent magnets, and a resinous material is locally pored into the gaps and solidified therein, so as to fix the permanent magnets to the rotor core (e.g., see Japanese Unexamined Patent Publication (Kokai) No. 5-83892 (JP-A-5-083892)).
In the rotor manufacturing method disclosed in JP-A-5-083892, grooves (i.e., gaps) extending in an axial direction are formed adjacent to and in communication with the respective magnet-retaining apertures of a rotor core, and a molten resinous material is injected into the grooves and solidified therein by an injection molding technique, so as to fix the permanent magnets in the magnet-retaining apertures. In the injection molding process, a mold including a cavity for accommodating the rotor core and an injection molding machine onto which the mold can be installed are provided, and the rotor core receiving the permanent magnets in the respective magnet-retaining apertures is supported firmly and securely in the cavity of the mold. In this state, the resinous material is injected into the grooves of the rotor core under pressure by using an injection molding machine.
In the rotor manufacturing method adopting the injection molding technique set forth in the above-described JP-A-5-083892, by providing only the mold including the cavity for accommodating the rotor core and the injection molding machine onto which the mold can be installed, it is possible to easily and quickly perform a work for fixing the permanent magnets to the respective magnet-retaining apertures of the rotor core and, therefore, advantages are obtained wherein complicated manual operations when using an adhesive and large-scale equipment when using an impregnant are eliminated. However, when the rotor core is disposed in the cavity of the mold, any of the permanent magnets may be subjected to a positional deviation in the respective magnet-retaining apertures of the rotor core, or may be removed from or fall out of the magnet-retaining apertures. If the permanent magnet falls out of the rotor core, a work for rearranging or inserting the permanent magnet into the corresponding magnet-retaining aperture is required and, thereby, a rotor manufacturing operation may be delayed. Further, if the permanent magnet is subjected to the positional deviation in the magnet-retaining aperture, the resinous material poured into the gap is solidified in this state so that the magnetic and mechanical balance in the whole body of the manufactured rotor may become out of order, which may result in the degradation of the rotational accuracy of the rotor and, thereby, in the deterioration of the operational reliability of the electric motor.